Schiering N, Kabsch W, Moore M J, Distefano M D, Walsh C T, Pai E F
Max-Planck-Institut für Medizinische Forschung, Abteilung Biophysik, Heidelberg, Germany.
Nature. 1991 Jul 11;352(6331):168-72. doi: 10.1038/352168a0.
Several hundred million tons of toxic mercurials are dispersed in the biosphere. Microbes can detoxify organo-mercurials and mercury salts through sequential action of two enzymes, organomercury lyase and mercuric ion reductase (MerA). The latter, a homodimer with homology to the FAD-dependent disulphide oxidoreductases, catalyses the reaction NADPH + Hg(II)----NADP+ + H+ + Hg(0), one of the very rare enzymic reactions with metal substrates. Human glutathione reductase serves as a reference molecule for FAD-dependent disulphide reductases and between its primary structure and that of MerA from Tn501 (Pseudomonas), Tn21 (Shigella), p1258 (Staphylococcus) and Bacillus, 25-30% of the residues have been conserved. All MerAs have a C-terminal extension about 15 residues long but have very varied N termini. Although the enzyme from Streptomyces lividans has no addition, from Pseudomonas aeruginosa Tn501 and Bacillus sp. strain RC607 it has one and two copies respectively of a domain of 80-85 residues, highly homologous to MerP, the periplasmic component of proteins encoded by the mer operon. These domains can be proteolytically cleaved off without changing the catalytic efficiency. We report here the crystal structure of MerA from the Gram-positive bacterium Bacillus sp. strain RC607. Analysis of its complexes with nicotinamide dinucleotide substrates and the inhibitor Cd(II) reveals how limited structural changes enable an enzyme to accept as substrate what used to be a dangerous inhibitor. Knowledge of the mode of mercury ligation is a prerequisite for understanding this unique detoxification mechanism.
数亿吨有毒汞化物散布在生物圈中。微生物可通过有机汞裂解酶和汞离子还原酶（MerA）这两种酶的相继作用，使有机汞化物和汞盐解毒。后者是一种与依赖黄素腺嘌呤二核苷酸的二硫化物氧化还原酶具有同源性的同型二聚体，催化反应NADPH + Hg(II)→NADP+ + H+ + Hg(0)，这是极为罕见的以金属为底物的酶促反应之一。人谷胱甘肽还原酶作为依赖黄素腺嘌呤二核苷酸的二硫化物还原酶的参考分子，在其一级结构与来自Tn501（假单胞菌属）、Tn21（志贺氏菌属）、p1258（葡萄球菌属）和芽孢杆菌属的MerA的一级结构之间，25% - 30%的残基得以保守。所有的MerA都有一个约15个残基长的C端延伸，但N端差异很大。虽然来自淡紫链霉菌的这种酶没有附加结构，但来自铜绿假单胞菌Tn501和芽孢杆菌属菌株RC607的酶分别有一个和两个拷贝的80 - 85个残基的结构域，与mer操纵子编码蛋白的周质成分MerP高度同源。这些结构域可通过蛋白水解作用切除而不改变催化效率。我们在此报道革兰氏阳性菌芽孢杆菌属菌株RC607的MerA的晶体结构。对其与烟酰胺二核苷酸底物及抑制剂Cd(II)的复合物的分析揭示了有限的结构变化如何使一种酶能够将曾经是危险抑制剂的物质作为底物接受。了解汞的连接方式是理解这种独特解毒机制的先决条件。
